Development of tethered dual catalysts: synergy between photo- And transition metal catalysts for enhanced catalysis

Article abstract of DOI:10.1039/d0sc02703k

While dual photocatalysis-transition metal catalysis strategies are extensively reported, the majority of systems feature two separate catalysts, limiting the potential for synergistic interactions between the catalytic centres. In this work we synthesise

Full text of DOI:10.1039/d0sc02703k

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Messerle, Chem. Sci., 2020, DOI: 10.1039/D0SC02703K.
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DOI: 10.1039/D0SC02703K
ARTICLE
Development of Tethered Dual Catalysts: Synergy Between Photo-
and Transition Metal Catalysts for Enhanced Catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Danfeng Wang,[a] Robert Malmberg,[a] Indrek Pernik,[a],[b] Shyamal K. K. Prasad,[c] Max Roemer,[a],[b]
Koushik Venkatesan,[a] Timothy W. Schmidt,[c] Sinead T. Keaveney[a],
* and Barbara A.
DOI: 10.1039/x0xx00000x
Messerle[a],[b]
*
While dual photocatalysis - transition metal catalysis strategies are extensively reported, the majority of systems feature
two separate catalysts, limiting the potential for synergistic interactions between the catalytic centres. In this work we
synthesised a series of tethered dual catalysts allowing us to investigate this underexplored area of dual catalysis. In
particular, Ir(I) or Ir(III) complexes were tethered to a BODIPY photocatalyst through different tethering modes. Extensive
characterisation, including transient absorption spectroscopy, cyclic voltammetry and X-ray absorption spectroscopy,
suggest that there are synergistic interactions between the catalysts. The tethered dual catalysts were more effective at
promoting photocatalytic oxidation and Ir-catalysed dihydroalkoxylation, relative to the un-tethered species, highlighting
that increases in both photocatalysis and Ir catalysis can be achieved. The potential of these catalysts was further
demonstrated through novel sequential reactivity, and through switchable reactivity that is controlled by external stimuli
(heat or light).
promote reactivity that is not possible using a single catalyst.
Significant advances in this emergent field have included the
development of a variety of dual-catalytic systems, such as
Introduction
Catalysts are essential tools in modern synthetic chemistry,
with 90% of industrial chemicals synthesised through a
sequence that involves at least one catalytic step.1, This
7,
8
combining: 1) metal catalysis and organocatalysis;5,
2)
photocatalysis and organocatalysis;9-14 and 3) photocatalysis
and metal catalysis.15-26 While dual catalysis has emerged as an
excellent synthetic platform for discovering new reactivity,
most reported dual catalytic systems feature individual
catalysts added as independent species to the reaction
2
extensive use of catalysts arises from their ability to make
synthetic processes more efficient, reducing energy
consumption and waste generation, as well as their ability to
promote otherwise unachievable transformations. Despite
their widespread use, there is a continuing need to advance
previous methodologies, with targeted catalyst design allowing
more efficient and practical chemical transformations to be
realised.
While the majority of well-established catalytic processes
utilise a single catalyst to facilitate the desired chemical
transformation, recently there has been a surge in interest in
merging different types of catalysis to permit new chemical
reactivity.3-6 In particular, there is a growing interest in
developing dual-catalytic systems, where cooperation
between different catalysts can increase reaction efficiency, or
Previous work: design
of new reac vity
through dual catalysis
Previous work: enhancing photocatalysis
through ‘heavy atom’ incorpora on
Intersystem
crossing
Promotes desired
intersystem crossing
pathway
S1
T1
Photocatalysis
heavy
atom
1O2
N
N
B
F
F
3O2
Heavy atom is typically a
halide or metal complex
S0
Transi on
metal
catalysis
Photocatalyst
I
Et3P Pt PEt3
N
N
F
F
I
I
B
In general: two separate
(untethered) catalysts,
limi ng synergis c effects
N
N
N
N
N
B
B
Rh
F
F
F
F
Cl
Cp*
Ref. 40
Ref. 37
Ref. 34
a. Department of Molecular Sciences
Macquarie University, North Ryde, NSW, 2109, Australia
This work: tethering
photo- and transi on
metal catalysts to
combine both
Dual role of Ir complex
photocataly c
enhancer and
independent cataly c
centre to promote
alternate reac vity
B
b. Current address: School of Chemistry
N
N
N
N
The University of Sydney, Sydney, NSW, 2006, Australia
Ir
c. ARC Centre of Excellence in Exciton Science, School of Chemistry
University of New South Wales, Kensington, NSW, 2052, Australia
* Prof. Barbara Messerle. E-mail: barbara.messerle@sydney.edu.au
* Dr. Sinead T. Keaveney. E-mail: sinead.keaveney@mq.edu.au
Electronic Supplementary Information (ESI) available: synthetic procedures;
characterisation data; crystallographic data; NMR spectra; details of the
photophysical, singlet oxygen quantum yield, cyclic voltammetry and XAS
measurements; catalytic procedures and control experiments.
N
N
L
L
B
F
F
Transi on
metal catalyst
approaches
Photocatalyst
Figure 1: Top: previous approaches to designing new reactivity using dual
catalysis, and more efficient photocatalysts. Bottom: the aim of this work.
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Chemical Science
Page 2 of 13
ARTICLE
Journal Name
properties make Ir(I) 2 and Ir(III) 3 ideal candiVdieawteAsrticalesOntlhinee
‘heavy atom surrogate’ attached to BDPD1O, Iw: 1h0e.1r0e39i/tDw0SilCl0a2c7t03aKs
both a photocatalytic enhancer and an independent catalytic
site. As there are many different ways to tether the catalysts,
three different linking modes were targeted to gain insight into
how the tethering mode affects catalytic cooperatively (Figure
2).
BArF
:
4
A
F3C
CF3
F3C
F3C
CF3
CF3
BArF
BArF
4
4
N
N
N
N
N
N
N
N
B
N
N
Ir
Ir
B
OC CO
Cl Cp*
F
F
Ir(I) 2
Ir(III) 3
BDP 1
F3C
CF3
BArF
BArF
B
C
4
4
Herein we report a series of tethered dual catalysts,
together with a thorough analysis of the structural and
physical properties of the new catalysts, providing particular
insight into the photophysical properties and catalytic
outcomes. This work clearly demonstrates for the first time
that chemically tethering heat and light activated catalysts
together can allow efficient dual catalytic strategies to be
developed, marking a substantial advancement in dual catalyst
design.
N
N
N
N
N
N
N
N
N
N
N
N
B
Ir
B
Ir
F
F
OC CO
Ir(I)-BDP SS 4
F
F
Cl Cp*
Ir(III)-BDP SS 7
BArF
BArF
4
4
N
N
N
N
N
N
N
N
N
N
F
F
F
CO
Cl
B
B
Ir
Ir
F
CO
Cp*
N
N
Ir(I)-BDP HH 5
Ir(III)-BDP HH 8
BArF
BArF
4
4
N
N
F
F
N
N
N
N
F
F
N
N
N
N
B
B
Results and discussion
N
N
Ir
Ir
OC CO
Cl Cp*
Design and synthesis of the bifunctional catalysts
Ir(I)-BDP HS 6
Ir(III)-BDP HS 9
As the target was to assess bifunctional catalysts featuring
different tethering modes, a modular synthetic approach was
desirable. In addition, an approach that is tolerant to a wide
range of functional groups could permit extension to other
ligand frameworks and photocatalysts in the future. As such,
Suzuki cross-coupling was utilised as the key step in
constructing our bifunctional catalysts. This approach is
particularly useful as there are numerous synthetic reports of
Figure 2: The parent catalysts on which the bifunctional catalysts are based (A);
the bifunctional catalysts featuring an Ir(I) (B) or an Ir(III) catalyst (C).
mixture, with comparatively little focus on single compounds
that feature two distinct catalytic sites.27-33 Recent examples of
these ‘bifunctional’ catalysts include
a photo-palladium
catalyst for Sonogashira cross coupling27 and a chiral copper
catalyst for enantioselective imine alkylation30
60
halogenated BDP 1 derivatives.36-38, As the BDP motif will
Chemically tethering different catalysts could permit
unique synergy between the catalytic centres, with this
approach anticipated to be particularly advantageous for
photocatalysis, as ‘heavy atom’ (e.g. halide or metal)
incorporation can enhance photocatalytic activity.34-39 In
particular, tethering metal complexes to the widely used 4,4-
difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) type dyes can
promote intersystem crossing (ISC) from the singlet to the
triplet excited state, leading to generation of reactive singlet
feature
a
halogen, the boronic ester substituted
bis(pyrazole)methane 12 was synthesised over two steps from
bis(pyrazole)methane. With these building blocks in hand,
Suzuki cross-coupling reactions were performed, generating
two bifunctional catalyst frameworks in good yield (Scheme 1).
These species have different tethering modes between BDP 1
and the ligand that will support iridium, with the connecting
modes termed as side-side (SS 13) and head-side (HS 14).
41
oxygen which is often key to photocatalysis (Figure 1).40,
While tethered BODIPY – metal complex species have been
applied as photocatalysts,42 therapeutics,43-46 gas sensing47-51
and as mechanistic probes,48, 52-54 their use in dual catalysis is
limited.27 As such, if a catalytically active ‘heavy atom’ unit was
tethered to BODIPY, this species could have the dual role of
enhancing photocatalysis, as well as providing an independent
catalytic site to promote alternative reactivity.
N
N
N
N
I
N
N
N
N
B
B
O
O
F
F
10
F
F
N
N
N
N
B
SS 13
73%
12
Pd(PPh3)4, K2CO3
THF:H2O (9:1),
N2(g), 85 °C, 48 h
N
N
N
N
F
F
F
F
N
N
N
N
B
Br
B
In this current work we explored this emergent area of
dual catalysis using bifunctional catalysts that feature a
photocatalyst tethered to a thermally activated transition
metal catalyst. In particular, the BODIPY-type photocatalyst,
BDP 1 (Figure 2) was chosen due to its excellent stability,
strong ground-state absorption and ease of modification.35, 39,
HS 14
76%
11
Scheme 1: Synthesis of the bifunctional catalyst frameworks SS 13 and HS 14
through Suzuki cross-coupling reactions.
As the synthesis of bis(pyrazole)methone 16 is well-known,61
the bifunctional catalyst framework for the head-head (HH 17)
tethering mode was synthesised through a cobalt catalysed
condensation reaction between compounds 15 and 16
(Scheme 2). The HH framework 17 was of particular interest
due to the sp3 hybridised CH moiety that disrupts conjugation
41
The iridium bis(pyrazole)methane based complexes Ir(I) 2
and Ir(III) 3 were chosen due to their high stability, including
air tolerance, ease of synthesis and their ability to promote
56
diverse
reactivity,
including
hydroamination,55,
58
59
dihydroalkoxylation57,
and hydrosilylation.56,
These
2 | J. Name., 2012, 00, 1-3
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ARTICLE
between the BDP and Ir centre, unlike the fully conjugated SS
13 and HS 14 frameworks.
Scheme 3: The coordination of Ir(I) or Ir(III) to the bifunctionalVliiegwanAdrt,icwleitOhnltihnee
synthesis of Ir(I)-BDP HH 5 and Ir(III)-BDP HH 8 shown as representative
examples.
DOI: 10.1039/D0SC02703K
Structural Characterisation
O
Nuclear Magnetic Resonance (NMR) spectroscopy, high
resolution mass spectrometry and elemental analysis
confirmed the formation of complexes 4-9. Crystals suitable
for X-ray crystallography were obtained for two ligands (14
and 17) and two complexes (5 and a derivative of 9 featuring a
BPh4 counterion) (Figure 3). The structural data, in
combination with the NMR spectral data, confirmed the
structures and tethering modes of these novel bifunctional
catalysts, and indicated that tethering BDP 1 to Ir(I) 2 or Ir(III) 3
led to no significant structural changes in the BDP or iridium
units regardless of the level of conjugation between the
iridium and boron centre (Tables S1 and S2).62-64 This is
important as it indicates that the photophysical or
electrochemical properties of 4-9, or changes in catalytic
activity relative to the two catalyst components, are not simply
due to structural differences that occur upon tethering.
Infrared spectroscopy allowed evaluation of the electronic
environment of Ir(I) in the carbonyl ligated species 4-6, relative
to the mononuclear complex Ir(I) 2. When moving from 2 to
the bifunctional species 4-6 there was a slight decrease in the
carbonyl stretching frequencies {2 (2100, 2035)65 > 4 (2098,
2035) > 5 (2093, 2029) > 6 (2091, 2028 cm-1)}. These data are
indicative of increased electron donation from the ligand to Ir
in the bifunctional catalysts. While this effect was minor, it
suggests that tethering Ir(I) 2 and BDP 1 results in increased
electron donation to Ir, likely due to electron transfer from the
BDP moiety.
CoCl2
O
N
N
N
N
N
N
toluene, N2(g)
F
F
B
N
N
N
N
+
N
N
130 °C, 48 h
B
F
F
HH 17
56%
16
15
Scheme 2: Synthesis of the bifunctional catalyst framework HH 17.
The final step of the synthetic strategy was coordination of the
Ir(I) or Ir(III) species to the bis(pyrazole)methane moiety in
compounds 13, 14 and 17, and formation of the cationic
iridium complex through addition of sodium tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate (NaBArF ). The same
4
methods were used for forming the Ir(I) and Ir(III) derivatives
for each framework, with representative reactions with 17
shown in Scheme 3. Overall, these synthetic strategies allowed
access to all six of the bifunctional catalysts 4-9.
N
N
N
N
N
N
F
F
B
1. [Ir(COD)Cl]2, NaBArF
CH2Cl2, N2(g), r.t., 3 h
,
HH 17
4
[IrCl2Cp*]2, NaBArF
,
4
CH2Cl2, N2(g), r.t., 3 h
2. CO(g), CH2Cl2,
r.t., 1 h
BArF
BArF
4
4
N
N
N
N
N
N
N
N
N
N
Ir
N
F
F
F
CO
CO
Cl
B
B
Ir
F
Cp*
N
Ir(I)-BDP HH 5
Ir(III)-BDP HH 8
81%
75%
Figure 3: Molecular structures derived from X-ray single crystal diffraction: a) HH ligand 17 (CCDC: 1955143); b) Ir(I)-BDP HH 5 (CCDC: 1955144), c) HS ligand 14
(CCDC: 1955141); d) an analogue of Ir(III)-BDP HS 9 containing a BPh4 counterion (see SI for details, CCDC: 1955142). Thermal ellipsoids are shown at the 50%
probability level. BArF4 and BPh4 counterions have been omitted for clarity.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Table 1: Photophysical and electrochemical properties of catalysts 1-9 and the ligand frameworks 13, 14 and 17.
View Article Online
DOI: 10.1039/D0SC02703K

abs / nm
E1/2 / V[f,g]
em
nm
[a]
/
kr /
knr /
air
air
(ε / x 105
F /
ns[a]
ΦF /
%
S
/ ns[d]
T
/ μs[d]
T
/
ΦISC
/ %[d]
ΦISC
Ered
/ V[f,h]
Complex
108 s-1
108 s-1
M−1 cm−1)
μs[e]
/ %[e]
[a,b]
[c]
[c]
ox
red
[a]
BDP 1
504 (1.00)
370 (0.03)
513
3.26
99
3.04
0.03
3.8
-
-
< 2
< 2
0.70
-1.72
-1.43[i]
-1.65,
-1.28
Ir(I) 2[j]
BDP 1 + Ir(I) 2
Ir(I)-BDP SS 4
Ir(I)-BDP HH 5
Ir(I)-BDP HS 6
Ir(III) 3[j]
503 (0.91)
507 (0.76)
509 (0.76)
514
526
519
516
3.29
3.58
3.03
2.57
67
63
23
48
2.04
1.76
0.76
1.87
1.00
1.03
2.54
2.02
3.6
3.8
3.6
2.1
-
>500
-
-
< 2
7.0
-
< 2
1.6
-
10.4
0.88
0.81
0.75
-1.69
-1.67
-1.72
-1.55
-1.61[i]
-1.58
-1.55
506 (0.21)
160
1.0
4.3
7.3
[k]
BDP 1 + Ir(III) 3
Ir(III)-BDP SS 7
503 (0.91)
510 (0.83)
514
533
3.26
2.70
65
49
1.99
1.81
1.07
1.89
3.7
2.8
-
-
< 2
7.2
< 2
7.3
>500
1.1
0.69
0.84
-1.69
-1.15
-1.91,
-1.62
Ir(III)-BDP HH 8
509 (0.81)
524
1.80
61
3.39
2.17
1.9
2.8
-
-
-
-
-
-
-
-
Ir(III)-BDP HS 9
SS 13
506 (0.47)
519 (0.61)
505 (0.85)
504 (0.83)
517
565
515
514
2.45
4.93
2.69
2.73
47
79
64
81
1.92
1.60
2.38
2.97
2.16
0.43
1.34
0.70
0.74
0.65
0.75
0.71
-1.71
-1.68
-1.66
-1.70
HH 17
HS 14
[a] Measured in toluene (1 x 10-5 mol/L) at 298 K. Uncertainty for abs and em: ± 1 nm. Uncertainty for F: ± 0.3 ns. [b] Absolute quantum yield measured with an
integrated sphere, uncertainty for ΦF: ± 5%. [c] Rates constants of radiative (kr) and non-radiative (knr) decay calculated using the formula kr = ΦF / F and knr =(1–
ΦF)/ F. [d] Singlet (s) and triplet (T) lifetimes, and intersystem crossing quantum yields (ΦISC) measured using transient absorption spectroscopy in toluene, under an
inert atmosphere. Uncertainty for s: ± 0.1 ns, and T: ± 0.1 μs. Uncertainty for ΦISC: ± 0.1 %. [e] T and ΦISC measurements in air. [f] Oxidation and reduction potentials
determined using cyclic voltammetry in CH2Cl2 (0.1 mol/L) using TBA-BArF4 as electrolyte, and calibrated using ferrocene. [g] Half-width potentials, assigned to the BDP
moiety. [h] Irreversible potential of the main cathodic peak reported. [i] Reported potential is a shoulder on the main BDP-centred reduction. [j] Ir(I) 2 and Ir(III) 3 have
a very weak absorption and no emission, thus limited photophysical data could be obtained. In addition, no clear oxidation wave was observed. [k] Absorption is
outside the wavelength range examined.
Figure 4: The UV-Vis absorption spectra (a-c, 1 x 10-5 mol/L) and normalised emission spectra (d-f) for BDP 1, catalysts 4-9 and ligands 13, 14 and 17 in toluene.
4 | J. Name., 2012, 00, 1-3
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View Article Online
DOI: 10.1039/D0SC02703K
Photophysical properties
Intersystem
crossing
1O2
To understand how the Ir centre influences the properties of
BDP 1, detailed photophysical investigations were performed
(Table 1). Firstly, analysis of the parent iridium catalysts 2 and
3 showed that these species have very low extinction
coefficients (ε) in the visible region (350-650 nm) and do not
emit light. This is important as it highlights that any differences
in the photophysical behaviour of BDP 1 and the bifunctional
catalysts 4-9 are not simply due to the Ir moiety acting as a
photocatalyst itself.
S1
Triplet
quenching:
T1
3O2
photocatalysis
Absorbance
Non-radia ve
decay
Fluorescence
Phosphorescence
S0
Figure 5: Simplified Jablonski diagram showing the possible excited state
pathways.
In general, the extinction coefficients of the compounds
were found to decrease on moving from BDP 1 to the
bifunctional catalysts (Table 1), indicating that catalysts 4-9 are
less effective at absorbing light than BDP 1, with the HS
tethered catalysts 6 and 9 being the weakest absorbers. The
absorption spectra of the HS 14 and HH 17 ligands have similar
profiles to BDP 1 (Figure 4), with a typical absorption near 504
radiative decay to S0 (Figure 5). Importantly, for photocatalysis
the ISC pathway is desired as this allows singlet oxygen to be
generated. As such, the preference for these competing
pathways is key for assessing photocatalytic potential.
As discussed above, all catalysts undergo fluorescent
decay, which is undesired for photocatalysis, with no
phosphorescence observed in our measurements. The
fluorescence lifetimes (F) were all found to be in the lower
nanosecond region (2-5 ns), with shorter lifetimes generally
observed for the bifunctional catalysts 4-9, relative to BDP 1
(Table 1). Importantly, the fluorescence quantum yield for BDP
1 was very high (ΦF = 99%), indicating that the desired ISC
pathway to T1 does not readily occur (<1%), making BDP 1 a
poor photocatalyst. Pleasingly, the fluorescence quantum
yields were significantly lower for catalysts 4-9 (23-63%), and
ligands 13, 14 and 17 (64-81%), indicating that undesired
fluorescent decay from S1 is significantly reduced for these
compounds. However, these lower ΦF could be due to an
increase in the desired ISC to T1,38, 48, 67 or undesired pathways
such as non-radiative decay from S1 to S0 or fluorescence
quenching due to intermolecular interactions, as observed for
the BDP 1 + Ir(I) 2 and BDP 1 + Ir(III) 3 mixtures.
The rate constant data show that in general the
bifunctional catalysts 4-9, and ligands 13, 14 and 17, have
lower rates of radiative decay (kr), and higher rates of non-
radiative decay (knr), relative to BDP 1. This indicates that non-
radiative decay pathways contribute significantly to the
photophysical behaviour of species 4-9. This is likely due to a
large extent of thermal energy loss through rotation of the
tethered Ir catalyst about the C – C bond that links BDP 1 and
Ir(I) 2/Ir(III) 3.47, 68-70 Overall, the increased non-radiative decay
from S1 to S0 observed for 4-9, relative to BDP 1, contributes to
their lower ΦF. To determine if increased ISC from S1 to T1 is
also contributing to these lower ΦF, transient absorption (TA)
spectroscopy was used to examine the excited states of BDP 1
and catalysts 4-9 (Table 1, Figures 6 and S24-S44).
nm, likely due to a ligand centred (LC) π(BDP)
π*(BDP)
→
transition.38, 39 The HS 14 and HH 17 ligands also had similar
absorption profiles to those of the HH and HS tethered
bifunctional catalysts 5, 6, 8 and 9, indicating that Ir
coordination doesn't affect absorbance maxima (λabs). In
contrast, the SS 13 ligand had λabs at 519 nm, with this
significant bathochromic shift, relative to BDP 1, suggesting
1
that the pyrazole moiety is involved in the LC π(SS) π*(SS)
→
transition for this ligand. In addition, the SS based bifunctional
catalysts 4 and 7 had a hypsochromic shift of 9-12 nm, relative
to the SS 13 ligand, suggesting that there is a significant
electronic interaction between the BDP moiety and the Ir
centre, which is consistent with that reported previously for
similar species.66
The emission spectra of BDP 1 and the bifunctional
catalysts 4-9 all feature one main band near 500 nm, with a
shoulder at lower energy that is most pronounced for catalyst
4 (Figure 4). In conjunction with the measured lifetimes (F),
1
this band can be assigned as fluorescence, likely due to a LC
π*(BDP) → π(BDP) transition. The emission maxima (λem) of the HH
and HS based catalysts 5, 6, 8 and 9 were comparable to BDP 1
and the ligands HS 14 and HH 17. However, a significant
bathochromic shift (52 nm) was observed for SS 13, relative to
BDP 1, suggesting that the pyrazole moiety is involved in the
1LC π*(SS) → π(SS) transition for this ligand. In addition, significant
hypsochromic shifts of 39 nm and 32 nm for λem were
observed for the SS tethered catalysts 4 and 7, relative to SS
13 (Figure 4 and Table 1). Once again, these data indicate that
the excited state of BDP 1 is altered the most when it is
tethered to the Ir moiety through the SS tethering mode.
Having established that BDP 1 and the bifunctional
catalysts 4-9 are effective at absorbing light, it was important
to consider the pathway(s) through which the excited species
decay. Following light absorption into the first singlet excited
state (S1), the excited photocatalyst can either: 1) decay to the
singlet ground state (S0) via radiative (fluorescence) or non-
radiative decay; or 2) undergo ISC to the first triplet excited
state (T1), followed by radiative (phosphorescence) or non-
The TA signal comprises a negative ΔOD (Optical Density) in
the 500-550 nm region, which could be caused by ground-
state bleaching (GSB) or stimulated emission (SE). A GSB signal
is indicative of there being molecules in an excited state, while
the presence of SE indicates a singlet state. The surface shown
in Figure 6a is decomposed into relative concentrations (6b)
and spectra (6c) by using a sequential model fit with two
exponentials. Figure 6c shows a comparison of the steady-
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state absorption, which mirrors the GSB, where it is evident
Journal Name
The triplet state lifetimes (T) were remarkably high for
View Article Online
DOI: 10.1039/D0SC02703K
that the first species has an additional lower energy region due catalysts 4 and 7 (>500 μs) with Ir(I)-BDP HS 6 also having a
to SE, which is not present in the triplet species.
long-lived triplet state (160 μs). To the best of our knowledge,
the highest reported T for a BDP-type compound is 539 μs,72
thus catalysts 4 and 7 represent one of the longest reported
triplet state lifetimes for BDP-type compounds. These long
triplet lifetimes are important for photocatalysis, as they
increase the likelihood of productive triplet energy transfer,
leading to 1O2 generation. This was confirmed through
transient absorption measurements in the presence of air,
which resulted in much shorter lifetimes (<2 μs), indicating
that quantitative triplet quenching by oxygen occurs (>99%).
Overall, these data suggest that Ir(I)-BDP SS 4 and Ir(III)-BDP SS
7 have more desirable photophysical properties than BDP 1
and catalysts 5, 6, 8 and 9, and thus should be more effective
photocatalysts.
Cyclic voltammetry.
To provide insight into electrochemical behaviour and the
potential for redox applications, cyclic voltammetry
measurements were performed on catalysts 1-9 and the
ligands 13, 14 and 17 (Table 1, Figure 7 and Figure S45). Cyclic
voltammograms (CVs) were recorded using a three-electrode
setup, with a glassy carbon working electrode, in a 0.1 M
solution of tetrabutylammonium BArF (TBA-BArF ) in
4
4
dichloromethane. TBA-BArF was chosen as the supporting
4
electrolyte as catalysts 2-9 all contain the BArF counterion,
4
and as TBA-BArF4 can allow multiple oxidation processes of the
analytes to be resolved due to its weakly coordinating
nature.73
ferrocene/ferrocenium couple.
All
potentials
are
reported
vs
the
The parent BDP 1 species was found to undergo well-
resolved one electron oxidation (0.70 V) and reduction (-1.72
70, 74
V) events, typical of BDP-type compounds.38,
Good
reversibility was observed, with the quotient of the anodic and
cathodic peak currents close to unity and an approximately
linear relationship between the peak currents and scan rate
(Tables S5 and S6). Similar reversible behaviour was observed
for the frameworks 13, 14 and 17, however better reversibility
Figure 6: The data from the transient absorption spectroscopy measurements for
Ir(I)-BDP SS 4, chosen as a representative example: a) the collected data, in the
absence of oxygen, using an excitation wavelength of 355 nm; b) fitted
exponential decay showing the time dependent behaviour of the singlet and
triplet excited states; and c) the species associated spectra.
The singlet lifetimes (S) were comparable to the previously was observed at higher scan rates. In addition, the oxidation
measured F, as expected (Table 1). A long-lived T1 was not and reduction potentials for 13, 14 and 17 were comparable to
detected for BDP 1, as anticipated based on the reported BDP 1, suggesting that the pyrazole moiety has no significant
lifetime of 0.02 μs in acetonitrile.71 Interestingly, triplet excited effect on the redox behaviour of the BDP 1 moiety (Table 1).
states were only detected for three of the six bifunctional While BDP 1 and the ligands 13, 14 and 17 exhibited
catalysts, indicating that the mode of tethering the Ir moiety to electrochemical reversibility, both the parent Ir(I) 2 and Ir(III) 3
BDP 1 plays an important role in populating the triplet states catalysts showed only weakly pronounced irreversible
(Table 1). The SS based bifunctional catalysts 4 and 7 were reduction events near -1.5 V, with no distinct oxidation events.
found to have the highest extent of ISC from S1 to T1, with ISC
quantum yields (ΦISC) of 7.0 and 7.2%, respectively. This is
important as it clearly demonstrates that tethering of Ir(I) 2 or
Ir(III) 3 to BDP 1 promotes the desired ISC pathway, with the SS
tethering mode being most effective. Interestingly, in the
air
presence of air the ΦISC for the Ir(I) based bifunctional
air
catalysts 4 and 6 increased, whereas ΦISC for Ir(III)-BDP SS 7
was unchanged.
6 | J. Name., 2012, 00, 1-3
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Ir(I)(CO)2 coordination. Overall, catalysts 4-9 exhibit reversible
View Article Online
BDP
1
electrochemical behaviour, that is domDinOaI:t1e0d.10b3y9/Dth0eSCB02D7P031K
fragment, highlighting their potential for use as catalysts for
redox processes. There were some interesting trends in
potentials observed when changing the Ir species and
tethering mode, demonstrating the possibility to tune the
redox potential of the catalysts through tethering modes.
0.00
-0.01
Ir(I)
2
0.001
-0.008
Ir(I)-BDP SS
Ir(I)-BDP HH
4
5
0.00
-0.01
-0.02
X-ray absorption spectroscopy.
0.00
-0.01
-0.02
To probe whether BDP is affecting the local electronic
structure of Ir in the bifunctional catalysts 4-9, X-Ray
Absorption Spectroscopy (XAS) measurements were
performed at the Ir L3 edge (11 – 12.5 keV) in transmission
mode. The XAS spectra indicate that there is a decrease in the
absorption edge energy on moving from Ir(I) 2 to the
bifunctional catalysts 4-6 (Figure 8). This decrease in edge
energy is smaller for Ir(I)-BDP HH 5 and Ir(I)-BDP HS 6 (<0.5
eV), with a more significant decrease observed for Ir(I)-BDP SS
4 (1 eV). This decrease in edge energy is characteristic of the
metal centre becoming more negative, with a decrease of 1 eV
being significant (for example, a difference of 1.6 eV between
Ir(IV) and Ir(III) has been reported76). Thus, the trend observed
suggests that BDP is transferring electron density to Ir(I),
making it less positive, with this effect most pronounced for
Ir(I)-BDP SS 4. This is consistent with the IR data, which also
suggests electron transfer from BDP to Ir(I) is occurring, and
our photophysical and electrochemical studies which suggest
that the greatest interaction between Ir and BDP occurs for
the SS tethering mode.
Ir(I)-BDP HS
6
0.00
-0.01
-0.02
Ir(III)
3
0.001
-0.008
0.01
0.00
-0.01
-0.02
Ir(III)-BDP SS
Ir(III)-BDP HH
7
8
9
0.00
-0.01
-0.02
-0.03
Ir(III)-BDP HS
0.0027
-0.018
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential V (Fc+/Fc)
Figure 7: Cyclic voltammograms of the parent compounds BDP 1, Ir(I) 2 and Ir(III)
3, and the bifunctional catalysts 4-9, measured in a 0.1 M solution of TBA-BArF
4
in dichloromethane under argon. Ferrocene was used as internal standard. Scan
rate: 100 mV/s.
Interestingly, no shift in edge energy was observed for the
Ir(III)-based catalysts 3 and 7-9 (Figure 9), suggesting that
transfer of electron density from BDP to Ir(III) does not occur.
Overall, these data indicate that the local electronic structure
of Ir(I) 2 becomes less positive when tethered to BDP, while
Ir(III) 3 is unaffected.
In general, the electrochemical behaviour of catalysts 4-9 was
dominated by the BDP 1 fragment, with reversible oxidation
and reduction events near 0.7 and -1.7 V, respectively. Poorly
defined reduction peaks due to the Ir(I) 2 or Ir(III) 3 moieties
were also present in the CVs (Figure 7). The splitting between
the major oxidation and reduction events varies from 2.3 to
2.6 V, which is comparable to reported alkyl,75 phenyl70 and
platinum34 substituted BDP derivatives. Comparison of the
ligand frameworks 13, 14 and 17 with the Ir(I)-based catalysts
4-6 indicate that coordination of Ir(I) to the ligand has no
significant effect on the reduction potential, however the
oxidation potential increased, with this increase most
pronounced for the SS framework (+0.23 V). This suggests that
the Ir(I) species affects the BDP moiety most when it is
tethered through the conjugated SS tethering mode, which is
consistent with our photophysical measurements.
Coordination of the Ir(III) moiety to the ligand frameworks
13, 14 or 17 generally led to more complex electrochemical
behaviour, with multiple reduction events observed for
catalysts 8 and 9. The Ir(III)-based catalysts 7-9 had higher
oxidation potentials than the corresponding ligand
frameworks, as seen for Ir(I), however the increases were less
pronounced. In addition, the maximum increase was now seen
for the HH catalyst 8 (+0.09 V). This data indicates that
coordination of Ir(III)ClCp* to the ligand frameworks leads to
different electrochemical behaviour than that seen upon
2.5
 Ir(I) 2
 Ir(I)-BDP SS 4
2
 Ir(I)-BDP HH 5
 Ir(I)-BDP HS 6
1.5
1
0.5
0
11150
11250
11350
11450
11550
11650
Energy / eV
Figure 8: The XAS spectra at the Ir L3 edge for the Ir(I) based complexes.
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Journal Name
fold increase in Φ relative to BDP 1. It should be noted that
Δ
View Article Online
DOI: 10.1039/D0SC02703K
2.5
2
greater increases in Φ would likely be observed if the heavy
Δ
atom was attached directly to the BDP 1 core,34, 35, 81 and thus
 Ir(III) 3
 Ir(III)-BDP SS 7
 Ir(III)-BDP HH 8
 Ir(III)-BDP HS 9
the smaller changes in Φ observed here are likely due to the
Δ
Ir centre being separated from BDP 1 by the tether. Despite
1.5
1
this, the variation in Φ between the bifunctional catalysts
Δ
indicates that both the tethering mode and the nature of the Ir
centre (Ir(I) 2 vs Ir(III) 3) affects singlet oxygen generation. This
is important as it clearly highlights the need to consider the
tethering mode when developing tethered dual catalysts.
To determine how singlet oxygen quantum yield affects
0.5
0
photocatalytic efficiency, the catalytic competency of
a
11150
11250
11350
11450
11550
11650
Energy / eV
representative selection of catalysts was examined using the
oxidation of benzylamine 18 as the model reaction. This
reaction was chosen as iodo-substituted BDP compounds have
previously been shown to effectively promote this oxidation
reaction.81 The complexes BDP 1, Ir(I)-BDP SS 4, Ir(I)-BDP HH 5
and Ir(III)-BDP SS 7 were tested as these catalysts cover a
Figure 9: The XAS spectra at the Ir L3 edge for the Ir(III) based complexes.
Catalytic investigations.
Photocatalytic applications of BDP-type dyes generally rely on
range of Φ . It was found that the bifunctional catalysts 4, 5
Δ
37, 46, 77, 78
the generation of reactive singlet oxygen,35,
thus
and 7 were significantly better photocatalysts than BDP 1
(Table 3). Control experiments in the presence of the singlet
oxygen scavenger 1,4-diazabicyclo[2.2.2]octane (DABCO)
confirm that singlet oxygen is involved in the reaction
mechanism for all catalysts (Table S8). Further control
reactions indicate that Ir(I) 2 and Ir(III) 3 are inefficient
photocatalysts; this is important as it demonstrates that
singlet oxygen quantum yields (Φ ) are a good indication of
Δ
photocatalytic potential. Incorporation of heavy atoms into
BDP can increase Φ due to enhanced intersystem crossing
Δ
from S1 to T1.35 This phenomenon was a key part of our
bifunctional catalyst design, as we sought to enhance the
photocatalytic competency of BDP 1 through incorporation of
a ‘heavy atom’, whilst also providing a separate catalytic site
that can facilitate complimentary catalytic reactivity. To
determine whether our bifunctional catalysts are superior
tethering
a
transition metal catalyst, that isn’t
photocatalytically active, to an organic photocatalyst can
significantly enhance photocatalytic activity.
singlet oxygen generators to BDP 1, Φ was determined for
Δ
Catalyst (1 mol%)
each catalyst under green LED irradiation (max wavelength =
510 nm) using 1,3- diphenylisobenzofuran as a singlet oxygen
trap (Table 2).36, 37, 79, 80 As alcoholic solvents are often used for
photocatalysis, the high boiling tertiary amyl alcohol solvent
was used.
N
NH2
2
x
t-amyl alcohol
green LED, air
19
18
Table 3: The efficacy of the different catalysts at promoting photocatalytic
oxidation of benzylamine 18 to the product 19.
Conversion to product 19 / %
Table 2: The singlet oxygen quantum yield (ΦΔ) for each catalyst, measured in tertiary
amyl alcohol. Average and error (half the range) of 2-3 replicate experiments reported.
See SI for experimental details (Figures S11-S23).
Catalyst
4 h
16 h
24 h
BDP 1
Ir(I)-BDP SS 4
Ir(I)-BDP HH 5
Ir(III)-BDP SS 7
Ir(I) 2
9 ± 3
23 ± 4
18 ± 1
18 ± 4
0
24 ± 5
59 ± 7
50 ± 3
46 ± 5
6
32 ± 7
79 ± 1
73 ± 5
72 ± 1
10
Complex
BDP 1
Φ / %
Δ
2.6 ± 0.1
3.2 ± 0.1
12.3 ± 0.4
3.6 ± 0.1
3.0 ± 0.2
3.9 ± 0.2
7.5 ± 1.7
1.2 ± 0.1
2.4 ± 0.2
BDP 1 + Ir(I) 2
Ir(I)-BDP SS 4
Ir(I)-BDP HH 5
Ir(I)-BDP HS 6
BDP 1 + Ir(III) 3
Ir(III)-BDP SS 7
Ir(III)-BDP HH 8
Ir(III)-BDP HS 9
Ir(III) 3
0
0
1
BDP 1 + Ir(I) 2
BDP 1 + Ir(III) 3
BDP 1 + NaBArF
12 ± 1
12 ± 1
8 ± 4
6 ± 3
42 ± 1
41 ± 3
24 ± 2
21 ± 2
59 ± 2
57 ± 3
42 ± 3
35 ± 2
4
BDP 1 + NaCl
Conditions: benzylamine (0.4 mmol), catalyst (0.002 mmol), additive, where
appropriate (0.002 mmol) 2,4,6-trimethoxybenzene (internal standard, 0.2
mmol), t-amyl alcohol (0.5 mL) in a vial open to air, with aliquots taken at
different time points. Average and error (half the range) of
experiments reported.
It was found that BDP 1 had a singlet oxygen quantum yield
of 2.6% in tertiary amyl alcohol, which is comparable to the
reported value of 1% for BDP 1 in dichloromethane.37
2 replicate
Pleasingly, Φ for the side-side tethered bifunctional catalysts
Δ
Comparison of the extent of conversion to the product 19 with
(Figure S53) gives a moderate correlation (R2 = 0.58),
indicating that singlet oxygen generation is not rate-
determining for this reaction, as seen for other processes
4 and 7 were higher than that for BDP 1, suggesting that
tethering Ir(I) 2 or Ir(III) 3 to BDP 1 can promote ISC. The most
efficient singlet oxygen generator was Ir(I)-BDP SS 4, with a 5-
Φ
Δ
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ARTICLE
Figure 10: Formation of the products 21 and 22 over time, monitored using in
situ 1H NMR spectroscopy. Conditions: diol 20 (0.2 mmol), catalyst (0.002 mmol),
involving singlet oxygen.37 This is supported by the catalytic
enhancements observed when using untethered mixtures of
‘BDP 1 + Ir(I) 2’ or ‘BDP 1 + Ir(III) 3’, relative to BDP 1, (Table 3);
toluene-d8 (0.5 mL) under argon. Conversion calculated relativVeietwo AtrhteiclestOarntliinneg
DOI: 10.1039/D0SC02703K
material 20.
this was unexpected as these mixtures have Φ similar to BDP
1 (Table 2). These data suggest that factors, other than simply
The activity of the Ir(III) based compounds 3 and 7 were
assessed for promoting the intramolecular hydroamination of
4-phenylbut-3-yn-1-amine 23 (Figure 11). Kinetic analyses
using in situ 1H NMR spectroscopy indicate that Ir(III)-BDP SS 7
can effectively promote hydroamination, with identical
reaction profiles obtained for Ir(III) 3 and Ir(III)-BDP SS 7. This
indicates that the Ir(III) 3 moiety remains catalytically active in
Ir(III)-BDP SS 7. Overall, these data indicate that the reactivity
of the Ir(I) 2 and Ir(III) 3 moieties are not inhibited when
tethered to BDP 1, with comparable, or better, catalytic results
obtained for the reactions considered. This is important, as it
demonstrates that the Ir centre is still able to act as a
competent catalyst when incorporated into the bifunctional
catalyst, highlighting the dual role of Ir as both a photocatalytic
enhancer and a unique reaction centre available to promote
alternate reactivity.
Δ
Φ , contribute to the synergistic effects observed when using
Δ
dual BDP 1 - Ir(I) 2 / Ir(III) 3 systems in photocatalysis. Control
reactions using a 1:1 mixture of BDP 1 and NaBArF suggest
4
that the BArF anion isn’t contributing to the enhancements
4
seen, while reactions using a 1:1 mixture of BDP 1 and NaCl
confirm that it is not simply a salt effect (Table 3). Therefore,
we postulate that the Ir centre is interacting with specie(s)
along the reaction coordinate, contributing to the catalytic
enhancements observed when using either tethered or
untethered BDP 1 - Ir(I) 2 / Ir(III) 3 systems (Table 3). While
mechanistic investigations into this phenomenon are ongoing,
it is likely that the effect of the Ir centre in the tethered dual
catalysts is two-fold; it increases Φ and is also directly
Δ
involved in the benzylamine 18 oxidation reaction mechanism.
Having established that the bifunctional species are
superior photocatalysts to BDP 1, attention will turn to the Ir(I)
and Ir(III) moieties. In this section the parent catalysts 2 and 3
were compared with the SS based complexes 4 and 7, as the SS
framework was most favourable for photocatalysis. The
catalytic reactivity of the Ir(I) moiety was assessed for
promoting the dihydroalkoxylation of 4-(2-(hydroxymethyl)-
phenyl)but-3-yn-1-ol 20 to produce products 21 and 22. Kinetic
N
NH2
Catalyst (1 mol%)
toluene-d8, 100 
C
23
24
100
90
80
70
60
50
40
30
20
10
0
1
analyses using in situ H NMR spectroscopy indicate that both
 Ir(III) 3
 Ir(III)-BDP SS 7
Ir(I) 2 and Ir(I)-BDP SS 4 are effective at facilitating this
reaction, and comparable product ratios were observed
(Figure 10). However, Ir(I)-BDP SS 4 was more efficient than
Ir(I) 2, indicating that tethering BDP to Ir(I) enhances the
catalytic reactivity of the Ir(I) moiety, possibly due to electron
transfer from BDP to Ir(I) as suggested by the XAS data. This is
important as it indicates that there are two types of synergy
between the catalytic centres in Ir(I)-BDP SS 4: Ir(I) 2 enhances
the photocatalyic ability of BDP 1 (Table 3) and BDP 1
enhances the catalytic reactivity of Ir(I) 2 (Figure 10).
0
1
2
3
4
5
6
7
8
Time / h
Figure 11: Formation of the product 24 over time, monitored using in situ 1H
NMR spectroscopy. Conditions: amine 23 (0.2 mmol), catalyst (0.002 mmol),
toluene-d8 (0.5 mL) under argon. Conversion calculated relative to starting
material 23.
OH
OH
O
Catalyst (1 mol%)
To further validate the bifunctional character of the novel
tethered catalysts 4-9, their ability to promote both sequential
and stimuli-responsive chemical reactivity was demonstrated
+
O
toluene-d8, 100 
C
O
OH
20
21
22
using Ir(I)-BDP SS
4 as a representative catalyst. This
O
100
90
80
70
60
50
40
30
20
10
0
bifunctional catalyst could promote a novel tandem reaction,
where the amine 25 first undergoes Ir(I) catalysed
intramolecular hydroamination to produce the intermediate
26, followed by BDP promoted photocalalytic oxidation to
generate product 27 in 60% isolated yield over two steps
(Scheme 4). This is important as it provides an alternative
synthetic approach to the medicinally important lactam
framework 27. In addition, there was a significant advantage
to chemically tethering the catalysts together, as
demonstrated by the much lower yield of the product 27 (17%)
obtained when using a mixture of BDP 1 and Ir(I) 2.
 Ir(I) 2
 Ir(I)-BDP SS 4
O
21
OH
O
22
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Time / h
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effective for photocatalysis. The superior photocatalytic ability
View Article Online
Sequential reactivity
of catalysts 4 and 7 was confirmed thDroOuI:g1h0.1s0i3n9g/Dle0tSCo0x2y7g0e3nK
H
H
N
N
Ir(I)-BDP SS 4 (1 mol%)
Ph
green LED
air, 18 h
O
Ph
quantum
yield
measurements
and
photocatalytic
NH2
toluene, 130 oC, 2 h
Ph
Ph
Ph
investigations. In addition, cyclic voltammetry indicated that
catalysts 4-9 exhibit reversible electrochemical behaviour that
is dominated by the BDP moiety. The oxidation and reduction
potentials varied depending on the nature of the Ir species and
tethering mode, highlighting the tunability of the bifunctional
catalysts’ redox potentials.
Ph
27, 60%
25
26
Scheme 4: The sequential hydroamination – oxidation reaction of compound 25 to
produce the lactam 27. Conditions: aminoalkene 25 (0.4 mmol), catalyst 4 (0.004
mmol), toluene (2 mL) under argon. Heated for 2 hours at 100 °C, then cooled to room
temperature, opened to air and irradiated for 18 hours. Isolated yield reported.
Importantly, Ir(I) and Ir(III) were shown to remain
catalytically active in the bifunctional catalysts 4 and 7 for
representative hydroamination and dihydroalkoxylation
reactions. This allowed the first demonstration of tethered
photo-transition metal dual catalysts to promote both
sequential and stimuli-responsive chemical reactivity. The key
fundamental insight into catalytic cooperatively presented in
this manuscript lays the groundwork for rationally designing
tethered photo-transition metal dual catalysts in the future,
and utilising these species to develop novel chemical
reactivity.
Scheme 5: The switchable reactivity of compound 25, where heat leads to generation
of compound 26 (right) and light irradiation produces compound 28 (left). Conditions:
aminoalkene 25 (0.4 mmol), catalyst 4 (0.004 mmol), toluene (2 mL). Either heated
under argon to give product 26, or irradiated under air to give product 28. Conversion
determined using 1H NMR spectroscopy, relative to the internal standard 2,4,6-
trimethoxybenzene (0.4 mmol).
Lastly, switchable chemical reactivity was demonstrated using
the amine 25 under different external stimuli, where use of
heat activated the Ir(I) moiety in Ir(I)-BDP SS 4, resulting in
hydroamination to compound 26. Conversely, light irradiation
activated the BDP moiety in catalyst 4, leading to
photocatalytic oxidation of 25 to the product 28 (Scheme 5).
This stimuli-responsive behaviour of the tethered dual catalyst
is significant, as controlling reactivity through external stimuli
is an emergent field as it is central to the development of
programmable and adaptive materials, and controllable
sequential reactions.82, 83 Overall, these are the first reported
examples of sequential and switchable reactivity using BDP-
based tethered dual catalysts.
Conflicts of interest
There are no conflicts to declare
Acknowledgements
The authors would like to thank the Chemical Analysis Facility
at Macquarie University, in particular Dr Nicole Cordina and Dr
Remi Rouquette. We also thank Dr Mohan Bhadbhade (UNSW
Australia) for the X-ray diffraction measurements. TWS and
SKKP are supported by the ARC Centre of Excellence in Exciton
Science (CE170100026). The XAS measurements were
performed on the XAS beamline at the Australian Synchrotron,
part of ANSTO (Project 14297), and we would like to thank Dr
Peter Kappen and Dr Jeremy Wykes for their assistance. DW
and RM acknowledge the support of Macquarie University
through the iMQRES scholarship program. STK acknowledges
the support of Macquarie University through the receipt of a
Macquarie University Research Fellowship (MQRF).
Conclusions
In summary, we have synthesised a series of novel tethered
dual catalysts that feature a BDP photocatalyst and a thermally
activated iridium catalyst, with synergistic interactions
between the catalysts examined using a range of techniques.
Absorption and emission spectroscopy revealed that the
excited state is centred on the BDP 1 moiety of the
bifunctional catalysts, with the excited state altered most
when the SS tethering mode is used. Interaction between BDP
and Ir(I) was also observed in the XAS data for catalysts 4-6,
with the greatest extent of electron transfer from BDP to Ir(I)
seen for the SS-tethered catalyst 4. Interestingly, the XAS data
suggest that electron transfer from BDP to Ir(III) does not
occur in catalysts 7-9.
Transient absorption spectroscopy indicated that tethering
Ir(I) 2 or Ir(III) 3 to BDP 1 can increase intersystem crossing
from the singlet to the triplet excited state, with long lived
triplet states located for Ir(I)-BDP SS 4, Ir(I)-BDP HS 6 and Ir(III)-
BDP SS 7. The highest extent of ISC and the longest triplet
lifetimes (>500 ns) were observed for the SS-tethered catalysts
4 and 7, suggesting that the SS tethering mode will be most
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View Article Online
DOI: 10.1039/D0SC02703K
A series of tethered BODIPY – ‘Ir complex’ dual catalysts were synthesised and thoroughly characterised.
Catalytic investigations demonstrated that tethering leads to enhancements in both photocatalysis and
thermally activated Ir catalysis. In addition, both sequential and switchable catalytic reactivity was achieved.
Transi on metal
Photocatalyst
catalyst
Ph
1O2
BArF
4
N
N
N
N
Enhanced
photocatalysis
Enhanced
Ir catalysis
N
N
B
Ir
3O2
F
F
OC CO
Sequen al reac vity
H
H
N
N
Ph
heat
light
Ph
O
NH2
Ph
Ph
Ph
Ph
Switchable reac vity
H
Ph
Ph Ph
Ph
N
Ph
Ph
light
heat
NH2
N
Ph
Ph